An analog to digital converter (ADC) is a device used to map continuous time signals to sampled time digital values. It is widely used in electronic systems.
An ideal ADC does not have any errors in its output. A sinusoidal input signal sampled by an ideal ADC does not have any harmonics in the frequency domain. In practice, however, ADCs are not ideal. Mismatches and nonlinearity of ADC elements introduces errors that distort the output of the ADC. The sampled sinusoidal signal has harmonics in the frequency domain, and the signal fidelity is degraded.
The quality of ADCs may be improved by trimming the ADCs to achieve desired transfer characteristics. FIG. 1 is a block diagram illustrating a system used to improve the quality of ADCs. A digitized sinusoidal reference signal is sent to a 16-bit digital to analog converter (DAC) 102. The DAC has good transfer characteristics and low noise. Its output is sent to a 16-bit ADC 104 that is under test. The ADC samples the output of the DAC, and sends its output to a filter 106 that performs a Fast Fourier Transform (FFT). If the output of the FFT is determined to have significant harmonic distortion, certain components of the ADC are trimmed. The trimming process usually changes the resistance or capacitance of the ADC, which may in turn reduce the harmonic distortion of the ADC.
There are several issues associated with the trimming technique. It takes time to test and trim the ADC, which increases the production cost. The design of trimming circuits also increases the complexity of the ADC. Furthermore, trimming the circuitry to adjust the transfer characteristics of the ADC for one frequency may adversely affect the transfer characteristics of the ADC for other frequencies. It would be desirable to have a technique that could correct the distortions in the ADC, without increasing the complexity of the circuitry or the production cost. It would also be useful if the technique could correct the distortions over the operating spectrum of the ADC.